Multi-point ignition system for shaped charges



May-13,1969 3,443,518

MULTI-POINT IGNITION SYSTEM FOR SHAPED CHARGES -Filed Sept. 26, 1967 vD. w. CROSS Sheet of 2 INVENTOR. DONALD W Ckoss BY I i 5M nroeuev:

United States Patent US. Cl. 102-24 2 Claims ABSTRACT OF THE DISCLOSUREA shaped charge formed of a housing having a main block of explosivewhich is provided with a lined generally-conical cavity. A plurality ofignition points are positioned about the explosive to ignite theexplosive in a predetermined manner to form a jet of optimum shape andvelocity.

Background of the invention This invention relates to shaped chargesand, more particularly, to the means of igniting the main block ofexplosive therein.

One of the principal commercial uses of shaped charges in industry isthe perforation of oil and gas wells. It has become accepted practice tomount one or more shaped charges either inside a suitable retrievable,fluid-tight, thick-wall cylindrical steel housing, or within individualexpendable, fluid-tight containers. In either case, a plurality ofshaped charges are usually employed, and they are positioned at suitablelongitudinally spaced-apart intervals, with the axes of the shapedcharges directed radially outward from the axis of the wellbore intowhich they are lowered. After the shaped charges are lowered to thedesired depth in the well, they are detonated from the surface. The jetformed by the shaped charge penetrates the casing and the formation toform a flow path whereby fluid in the formation may flow into thewellbore. The effectiveness of the shaped charge depends upon the depthof penetration and a debris-free perforation.

Shaped charges for the above-described purposes are usually constructedof a housing containing a body of high-explosive material which isformed with an outwardly-facing concavity which may take variouspredetermined shapes for different purposes, but for the usual wellperforating service is normally of conical form. The outwardly-facingsurface of such concavity is provided with a relatively thin linerformed of suitable material. In order to detonate the shaped charge, abooster, customarily placed at the axial rearward end of the explosivecharge, is ignited. The booster produces a shock wave WhlCh detonatesthe explosive charge as it travels through the explosive. As the wave ofdetonation of the main charge progresses from the point of initiation,it encounters the liner and the cavity. A progressive collapsingdisintegration of the liner occurs with the wave front. This results inthe material forming the liner converging at the longitudinal axis ofthe charge, resulting in the formation of a highspeed particle-ladenjet. The tip of the jet has a velocity of about 30,000 ft./sec.;however, a portion of most common liners is not disintegrated andfollows behind the faster jet stream as a relatively slow-moving slugwhich may only have a velocity of 8,000 ft./sec. The perforation is madeby the resulting particle-laden stream of high velocity, hightemperature and pressure, fluid jet. The slug is detrimental and manyspecial types of liners have been developed in an effort to reduce orcompletely eliminate it.

It is the purpose of the present invention to develop a more effectivemeans of detonating the explosive charge in order to form an optimum jetof maximum velocity.

Summary of the invention The shaped charge of the present invention isgenerally similar in construction to the shaped charges now in use, thatis, a main block of high-explosive is contained in a case and a linedconcavity is located in the face of the explosive block. However,instead of having a single ignition point located at the rear apex ofthe block of explosive, there are a plurality of ignition points spacedin a predetermined manner about the block of explosive and an ignitionpath runs from a primary ignition area to the plurality of ignitionpoints to detonate the block in a predetermined manner, therebyproviding for the formation of pressure forces at the correct instant oftime which will optimize the jet.

Brief description of the drawings FIG. 1 is a cross-sectional view of ashaped charge constructed in accordance with the present invention,

FIG. 2 is a plan view showing the ignition paths,

FIG. 2a is a cross sectional isometric of a shaped charge illustratingan alternate form of the multi-ignition system,

FIG. 3 is a diagram showing the detonation paths of prior art charges,

FIG. 4 is a diagram showing the lost energy and detonation of prior artcharges,

FIG. 5 is a diagram showing the formation of a jet of the prior artcharges,

FIG. 6 is a diagram showing the detonation of the proposed charge,

FIG. 7 is a diagram showing the formation of a jet by the proposedcharge,

FIG. 8 is a diagram showing the formation of the jet and slug in priorart charges,

FIG. 9 is a diagram showing the formation of a jet in the proposedcharges,

FIG. 10 is a diagram showing the formation of hyperpressure areas,

FIG. 11 is a diagram showing the time of detonation of a prior artcharge and the proposed charge,

FIG. 12 is a cross sectional view of an alternative form of themulti-ignition system.

Description of the specific embodiment Referring now to the drawings, itcan be seen that the shaped charge of the present invention is formed ofa cup-shaped housing 10 in which is located a block of explosive 12having a cavity 14 in its front face. A liner 16 covers the face of thecavity. In the above respects, the shaped charge is similar to those inthe prior art. However, instead of having a single ignition pointlocated at the rear apex of the block of explosive 12, the shaped chargeof the present invention is provided with a multi-point ignition systemwhich ignites the main block of explosive in a predetermined manner.

Accordingly, the case 10 is provided with a primary ignition area 18which may be ignited by a Primacord or other well-known ignition means.Forward of primary ignition area 1 8 is .an inner case 22 which isprovided with a series of paths 24 extending from the primary ignitionarea 18. An explosive, having a known rate of detonation, may fill thepaths 24. The inner case has a series of apertures 2 6 which are incontact with main block of explosive 12. The length of paths 2 4 ispredetermined so that upon ignition of primary area 18, the explosive inpaths 24 is ignited which results in the ignition of the explosive atapertures 26 which results in the ignition of the main block 12 in apredetermined manner. Any method for controlling the rate of detonationof the explosive block -12 may be utilized. For example, the length ofthe ignition paths 24, the thickness, density and 3 shape of theexplosive in the paths or the main block may be varied.

The length of each ignition path 24 from the common primary ignitionarea 18 to the individual ignition points 26 on the main explosivecharge is predetermined. By varying the lengths of paths 24 and knowingthe rate of detonation of the main explosive charge and the explosive inignition paths 24, a firing order can be developed which will form a jetof an optimum shape and of greater velocities than prior art charges. Inorder to accomplish this, the ignition of explosive charge 12 by themultiple ignition points preferably occurs before the detonation wave ofthe main explosive charge reaches such location. As a result,hyper-pressure areas are formed by the augmentation of the multipleshock waves which increase the velocity of the collapsing liner and theresulting jet.

FIG..2 shows a typical ignition system which includes not only theprimary paths shown in FIG. 1, but also various branches extendingtherefrom. As can be appreciated, with such a multi-point ignitionsystem, there will be an overall superior form of ignition of the mainblock of explosive and, as a result, a more effective total detonationof the main block of explosive, and, as a consequence, an optimizationof the jet. While the ignition system illustrated shows paths filledwith high explosive, other types of ignition systems such as anelectrical network or optical fibers with light-sensitive explosive maybe used to provide the multi-point ignition. FIG 2A illustrates a shapedcharge of the present invention having multi-ignition paths 30 in a mainhousing 32. As in the case of the shaped charge shown in FIGS 1 and 2,there is a main explosive charge 12 having a cavity 14 in its front facewhich is provided with a liner 16. The ignition paths 30 extend from aprimary ignition area 34 to multiignition points 36 located on the rearsurface of main explosive 12.

The ignition points 26 and 36 lie in rings and columns similar to theintersections of the longitudinal and latitudinal lines on a globe. Thedistance between neighboring points preferably approximate the thicknessof the main explosive charge. The hyper-pressure area that is formed bythree converging detonation waves, simultaneously initiated, is shapedin the form of a pyramid having concave sides with a base on the liner.With such a construction, the center of force will act on the liner atthe centeroid of the pyramid.

In a shaped charge, the liner collapses due to high pressures from thedetonation of the explosive charge and not the shock wave. However, thepressures formed and the shock wave are related since the shock wave isindicative of the pressure forces. If a pressure gradient is developedfor detonation, it will indicate that the highest pressure is next tothe shock wave and the pressure decreases further back from the shockwave. Therefore, the direction of the collapse of the liner due topressure approximates the collapse as if it is caused by the shock Wave.This phenomenon occurs because the pressure is not uniform over thesurface of the collapsing liner because the detonation is not static butdynamic.

In the customary shaped charge, as previously mentioned, an ignitionpoint or booster is located at the apex of the explosive charge. Thisbooster is ignited and it, in turn, results in a detonation of theexplosive charge. The direction of detonation is in the form of aspherical shock wave or a wave normal to the axis of the charge, seeFIG. 3, where the vector V indicates an assumed direction of thepressure force and D indicates the direction of detonation. Under staticconditions, vector V would be perpendicular to the liner. However, underdynamic conditions, it would have a tendency to move from theperpendicular direction towards the direction of detonation.Accordingly, there is a loss in velocity, see 'FIG. 4 where V indicatesthe vector of the pressure force, vector V indicates the liner collapseand the vector V indicates Waste. The two vertical components of thevector of liner collapse V cancel each other out as the jet is formedand moves horizontally, see FIG. 5 Where the two vectors V represent theliner collapse and V represents the vector on the jet. One of thepurposes of the present design is to eliminate this waste since,although the detonation is dynamic, the pressure is being applied in adirect line to the desired direction of travel of the liner, see FIG. 6Where the vector V indicates the direction of detonation in the proposedcharge. Accordingly, as can be seen in FIG. 7, the formation of the jetis caused by the converging of the two vectors V In this design thevertical components of vectors V cancel each other and the jet will havegreater velocity since there is only one waste of energy and not two.

The proposed design presents a method of eliminating the slug in part orwhole. Although in some prior art shape charges efforts have been madeto eliminate the slub by other means, the source of the problem has notbeen eliminated. Therefore, in the prior art shape charges part of themass of the liner is wasted as well as the energy that is used toeliminate its formation. If the tendency to form the slug is eliminatedthen this mass and energy could be put to useful work.

The source of the problem is that the shock wave is ahead of thecollapse of the liner. If, on the other hand, the liner collapsed at arate exceeding the rate of detonation of the explosive then the linerwill collapse in a progressively forward direction, see FIG. 9. In suchcase, there is no shearing of the back part of the collapsed liner andno slug. Moreover, the jet and the collapsed liner are constantlypulling ahead of the detonation; therefore, there is no tendency for thecollapsed liner to shear and form a slug; all of the liner is formedinto a high velocity jet. The liner is sufiiciently plastic that it willmake the sharp bend at the instant of collapse.

A significant feature of multi-point detonation is the formation ofhyper-pressure areas due to the concentration of pressure caused byseveral shock waves coming together. The formation of these highpressure areas is dependent upon the spacing of the ignition points andthe thickness of the explosive charge.

Preferably, the distance between an ignition point and its neighboringpoint should be in a range approximating the thickness of the explosivecharge. The correct distance is one that will create the maximumvelocity of the jet by the most advantageous balance of all variables.If this distance is too great the shock wave will rupture the liner inan incorrect manner and the liner will not collapse properly. The closerthe points of ignition are to a respective neighbor, the moreintensified the hyper-pressure area will be. If the two points aredetonated simultaneously or in rapid sequence the two shock waves areapproximately converging on area ha, see FIG. 10. Since the detonationis a violent expansion of gas due to the rapid burning of the explosiveand the two detonation waves are converging, tremendous pressures willbe developed in the area between them which is designated ashyperpressure area he in FIG. 10. These pressures will collapse theliner at a rate faster than the rate of detonation.

Another significant feature of the multi-point ignition system is thatit detonates the shaped charge more quickly than the single pointmethod, see FIG. 11 where the upper side illustrates the direction ofdetonation of the prior art, and the lower side illustrates thedirection of detonation of the present invention. It can be seen thatthe detonation time is that required for the detonation to move from Ato B, for prior art charges, and from A to B in the proposed charge.Therefore, the explosion will occur AB/AB times faster. Assuming thatthe given amount of explosive in both systems possess the same amount ofpotential energy, the proposed design will release it AB/AB' timesfaster. Assuming the surface area of the liner is the same for bothsystems, a greater pressure during the time of detonation will bedeveloped for the multipoint system. This will likewise create a greateramount of work available for perforating the formation. Since thereleased kinetic energy is increased for an instant, the velocity of thecollapse of the liner is greater, and this increase approaches inaccordance with the following formula:

Therefore, all else being equal, the velocity of the jet should increaseby approximately times due to this advantage. In this connection, ratherthan having the inner case with its plurality of apertures and ignitionpaths, the rear surface of the explosive charge or selected portionsthereof may be provided with photosensitive explosive 40, see FIG. 12,and the charge spaced from the housing. The wall of the housing may beprovided with a reflective surface 42 serving as the ignition from theprimary ignition area to the multi-ignition points on the charge. Theprimary ignition area may be provided with a high-intensity light source44. Therefore, ignition of the source 44 will cause ignition of theentire surface of the explosive charge in a predetermined manner. Suchignition will result in a more complete detonation of the charge andthereby optimize the formation of the jet.

The proposed design tends to focus the shock wave which may be useful inrupturing the perforated hole. Since the present design presents ascheme for gaining a more effective use of the available energy, equalpenetration of a formation could be obtained by using a smaller charge.This would mean less casing and gun damage for a hole of the samepenetration.

A sequenced detonation has one additional value. It allows for theelongation of the jet. If all points are detonated simultaneously, theliner is blasted into a mass the length of which cannot exceed thelength of the liner. However, in the sequenced detonation the jet isallowed to elongate some just as it does in the prior art charges. Thisgives greater penetration of the formation and less damage to the plugof the gun and the casing. For example, if the jet is moving at avelocity three times the rate of detonation, the apex will move threetimes the distance I in the time to detonate the distance I.

t=0, collapse of apex of liner, velocity 3 times that of liner 1:],detonation complete, apex (tip of jet)) moved 3 times the length of thedetonation (as abbreviated due to the firing order) plus the distancefrom each point on the liner to the longitudinal axis.

For a liner 1 /2" long with 6 rings of ignition points equally spacedthe jet would be approximately 2 in length. The entire jet is moving atthe same velocity. In the prior art shaped charges the back part of thejet is moving at a velocity of about A of that of the tip. For thisreason, the present charges have more elongated jets,

but the back part is not effective since it is moving at a relativelyslow velocity.

As can be seen from the foregoing, the shaped charge of the presentinvention is provided with multi-point ignition which initiates thedetonation of the main explosive charge in a predetermined manner. As aresult, the detonation of the explosive block is improved and there isan optimizing of the jet. Moreover, if the ignition is in apredetermined sequence, there is developed hyper-pressure areas whichwill collapse the liner at a rate faster than the rate of detonation andform a jet of optimum shape and velocity.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

I claim:

1. A shaped charge comprising a cup-shaped housing, a cup-shaped maincharge of explosive in said housing, a cavity in the face of saidexplosive charge, a cup-shaped liner covering said cavity, a primaryignition area at the rear of said housing, a plurality of longitudinallyspaced rings of ignition points on the rear surface of the explosivecharge, the ignition points of each ring being equally spaced and meansfor igniting each ring of ignition points in sequence with the ringadjacent the primary ignition area being ignited first and then the nextclosest, and the next until all rings have been ignited and the ringfarthest from the primary ignition area being the last to ignite withthe time interval between the ignition of each ring being such that thering of ignition points adjacent the primary ignition area is ignitedbefore the shock wave from the portion of. the main charge ignited bythe primary ignition area reaches the first ring and each subsequentring being ignited after the ring adjacent to it but before the shockwave from the adjacent ring reaches it to cause the shock wavestraveling from each ignition area to converge and produce areas betweenthem of htyperpressure that will force the liner to collapse ahead ofthe shock wave from each successive ignition point and move forward withthe forward moving jet formed by the progressive firing of the ignitionpoints, said shaped charge being further characterized by having eachring of ignition points spaced to create hyperpressure areas that are sospaced that the liner will be collapsed inwardly without shearing theliner.

2. The shaped charge specified in claim 1 wherein each ring of ignitionpoints and the ignition points of each ring are spaced from each otherat a distance approximating the thickness of the main explosive charge.

References Cited UNITED STATES PATENTS 2,763,210 9/1956 Church et al.102-24 2,856,850 10/1958 Church et al 102-24 3,170,402 2/1965 Morton etal. 3,311,055 3/1967 Stresau, Jr. et al. 3,325,317 6/1967 Voigt.

FOREIGN PATENTS 1,051,708 2/ 1959 Germany. 1,172,591 6/ 1964 Germany.

VERLIN R. PENDERGRASS, Primary Examiner.

